#include <bits/stdc++.h>

#define in read()
#define fi first
#define se second
#define pb push_back
#define rep(i, x, y) for(int i = (x); i <= (y); i++)
#define per(i, x, y) for(int i = (x); i >= (y); i--)

using namespace std;

typedef long long ll;
typedef double db;
typedef vector < int > vec;
typedef pair < int , int > pii;

int read() {
    int x = 0; bool f = 0; char ch = getchar(); while(!isdigit(ch)) f |= ch == '-',ch = getchar();
    while(isdigit(ch)) x = x * 10 + (ch ^ 48),ch = getchar(); return f ? -x : x;
}

const int N = 1e5 + 10;

int n;
db a[N], b[N], s[N], v[N];

int main() {
#ifndef ONLINE_JUDGE
    freopen("1.in","r",stdin);
#endif
    n = in;
	rep(i, 1, n) cin >> a[i] >> b[i], s[i] = s[i - 1] + a[i] / b[i];
	db t = s[n] / 2; 
	rep(i, 1, n) {
		if(t < s[i]) {
			db ans = 0;
			rep(j, 1, i - 1) { ans += a[j]; }
			ans += (t - s[i - 1]) * b[i];
			printf("%.7lf\n", ans);
			return 0;
		}
	}
}
